Complementary metal oxide semiconductor (CMOS)

Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits.

CMOS

Complementary
metal–oxide–semiconductor (CMOS) is a
technology for constructing integrated circuits. CMOS technology is used in
microprocessors, microcontrollers, static RAM, and other digital logic
circuits. CMOS technology is also used for several analog circuits such as image
sensors, data converters, and highly integrated transceivers for many types of
communication. Frank Wanlass patented CMOS in 1967 (US patent 3,356,858).

CMOS is
also sometimes referred to as complementary-symmetry metal– oxide–semiconductor
(or COS-MOS). The words "complementary-symmetry" refer to the fact
that the typical digital design style with CMOS uses complementary and
symmetrical pairs of p-type and n-type metal oxide semiconductor field effect
transistors (MOSFETs) for logic functions.

Two important
characteristics of CMOS devices are high noise immunity and low static power
consumption. Significant power is only drawn while the transistors in the CMOS
device are switching between on and off states. Consequently, CMOS devices do
not produce as much waste heat as other forms of logic, for example
transistor-transistor logic (TTL) or NMOS logic, which uses all n-channel
devices without p-channel devices. CMOS also allows a high density of logic
functions on a chip. It was primarily this reason why CMOS won the race in the
eighties and became the most used technology to be implemented in VLSI chips.

The
phrase "metal–oxide–semiconductor" is a reference to the physical
structure of certain field-effect transistors, having a metal gate electrode
placed on top of an oxide insulator, which in turn is on top of a semiconductor
material. Aluminum was once used but now the material is polysilicon. Other
metal gates have made a comeback with the advent of high-k dielectric materials
in the CMOS process, as announced by IBM and Intel for the 45 nanometer node
and beyond.

Sequential Logic Basics

Unlike
Combinational Logic circuits that change state depending upon the actual
signals being applied to their inputs at that time, Sequential Logic circuits
have some form of inherent "Memory" built in to them and they are
able to take into account their previous input state as well as those actually
present, a sort of "before" and "after" is involved. They
are generally termed as Two State or Bistable devices which can have their
output set in either of two basic states, a logic level "1" or a
logic level "0" and will remain "latched" indefinitely in
this current state or condition until some other input trigger pulse or signal
is applied which will cause it to change its state once again.

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